So, none of the photos you provided are showing up anymore (for me at least), and thats the one thing I've been looking for. I want to install a tachometer to my base model 1991 d21 4cyl but have no idea how.

The 12v wire has to be tapped into something that has power with the key on. I spliced into the cigarette lighter wire. You'll need a multimeter to determine which is the positive side.

For the backlight I tapped into the wire for the instrument cluster coming off the dimmer switch. Again, you'll want the multimeter to make sure you got the right wire. The output voltage should change linearly as you rotate the dial. That way the tach backlight will dim with the rest of the lights (well it should, my dimmer doesn't work lol).

For the tach signal I spliced into the signal wire coming off the back of the ignition coil. It's the blue wire. You will need to run a wire through the firewall, I just ran it through the same hole as the hood release cable.

The ground is easy. If you remove the driver's side kick panel there are already a couple existing ground screws you can use.

Then I taped up the wires to make it nice and neat. Let me know if you need a picture or two.